Securities - Details of Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 14,288	$ 13,365	$ 12,306
Gross Unrealized Gains	151	132	138
Gross Unrealized Losses	63	137	199
Fair Value	14,376	13,360	12,245
Held to maturities, Amortized Cost	4,936	5,015	4,997
Held to maturities, Gross Unrealized Gains	27	16	10
Held to maturities, Gross Unrealized Losses	23	57	96
Held to maturities, Fair Value	4,940	4,974	4,911
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	27	29	28
Gross Unrealized Gains	1	3	4
Fair Value	28	32	32
Held to maturities, Amortized Cost	20	20	20
Held to maturities, Fair Value	20	20	20
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14	22	26
Gross Unrealized Gains	1	1	1
Fair Value	15	23	27
Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,938	11,310	10,096
Gross Unrealized Gains	127	96	109
Gross Unrealized Losses	62	136	196
Fair Value	12,003	11,270	10,009
Held to maturities, Amortized Cost	4,299	4,755	4,977
Held to maturities, Gross Unrealized Gains	24	15	10
Held to maturities, Gross Unrealized Losses	23	57	96
Held to maturities, Fair Value	4,300	4,713	4,891
Other Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,309	2,004	2,156
Gross Unrealized Gains	22	32	24
Gross Unrealized Losses	1	1	3
Fair Value	2,330	2,035	$ 2,177
Held to maturities, Amortized Cost	617	240
Held to maturities, Gross Unrealized Gains	3	1
Held to maturities, Fair Value	$ 620	$ 241